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2013-08-06T19:00:41
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2013-08-06T19:00:41
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Java has no unsigned numbers (<code>char</code> is 16bit unsigned but it's not a number and math with <code>char</code> will always result in implicit casts to <code>int</code>) If you read 2 bytes of unsigned data into a <code>short</code> and want to see values in range from 0-65535 (instead of -32768 - 32767) you'll have to use a type that can have values in that range. In case of 16bit <code>short</code> next bigger one is 32bit <code>int</code>. The conversion that does the trick is <pre><code>short signed = ...; int unsigned = signed & 0xFFFF; </code></pre> Assuming <code>signed</code> has a value of <code>0xFFFF</code> this is what happens: <pre><code>short signed = -1; // FFFF on byte level </code></pre> The expression <code>signed & 0xFFFF</code> contains a <code>short</code> and an <code>int</code>. <code>0xFFFF</code> is a literal integer type number which when found in Java source is considered <code>int</code>. You could make it <code>long</code> by changing it to <code>0xFFFFL</code> (you would need that if you want to convert unsigned <code>int</code> to <code>long</code>). Since the <code>&</code> operator needs both sides in a common type Java will silently convert the smaller one. <pre><code>int stillSigned = (int) signed; // hidden step </code></pre> It will still have the exact same value of -1 since that's what it was before when looking at it unsigned but it is changed on bytelevel to <code>0xFFFFFFFF</code>. Now the bit-manipulation is applied to remove all the added <code>FF</code>s <pre><code>int unsigned = stillSigned & 0xFFFF; </code></pre> and you end up with <code>0x0000FFFF</code> on byte level and can finally see the value of 65535. Since you happen to have 16bit values you can use <code>char</code> and simply cast it to <code>int</code>. <pre><code>char value = ...; int unsigned = value; </code></pre> But above approach works for any unsigned conversion: <code>byteValue & 0xFF</code>, <code>shortValue & 0xFFFF</code>, <code>intValue & 0xFFFFFFFFL</code> <hr> The next thing you should do is not to use a simple <code>InputStream</code> to do <pre><code>SerialPort device = SerialPort(file, baud, flags); InputStream in = device.getInputStream(); byte[] buffer = new byte[384]; in.read(buffer); </code></pre> Reason is that <a href="http://docs.oracle.com/javase/7/docs/api/java/io/InputStream.html#read%28byte%5b%5d%29" rel="nofollow">InputStream#read(byte[])</a> is not guaranteed to read all the bytes you want in your buffer. It returns you the number of bytes it has read or <code>-1</code> if the stream is done. Manually writing code that ensures you have a filled buffer is nasty but there is a simple solution: <a href="http://docs.oracle.com/javase/7/docs/api/java/io/DataInputStream.html" rel="nofollow">DataInputStream</a> <pre><code>SerialPort device = SerialPort(file, baud, flags); DataInputStream in = new DataInputStream(device.getInputStream()); byte[] buffer = new byte[384]; in.readFully(buffer); </code></pre> <code>DataInputStream</code> has very nice functionality that you could use: <pre><code>SerialPort device = SerialPort(file, baud, flags); DataInputStream in = new DataInputStream(device.getInputStream()); int unsignedShort = in.readUnsignedShort(); </code></pre> Another way to get different numbers out of <code>byte[]</code> data is to use <code>ByteBuffer</code> since that provides methods like <code>.getShort()</code> <pre><code>SerialPort device = SerialPort(file, baud, flags); DataInputStream in = new DataInputStream(device.getInputStream()); byte[] buffer = new byte[384]; in.readFully(buffer); ByteBuffer byteBuffer = ByteBuffer.wrap(buffer); while (byteBuffer.hasRemaining()) { int unsigned = byteBuffer.getChar(); System.out.println(unsigned); } </code></pre>
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1. POReading serial information in Java
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1. CONote that DataInputStream assumes big-endian byte ordering and that this cannot be changed.
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2. COThat's true. E.g. http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/LittleEndianDataInputStream.html would help then. Or the `ByteBuffer` way and specifying the correct `ByteOrder`.
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